This invention relates to hydraulic systems for controlling a plurality of actuators such as hydraulic cylinders which are found, for example, in earth moving equipment such as excavators and cranes. In such a system, it is conventional to provide a pilot operated control valve for each actuator which is controlled by a manually or fluid or electric operated controller through a pilot hydraulic circuit. The control valve functions to supply hydraulic fluid to the actuator to control the speed and direction of operation of the actuator.
In U.S. Pat. Nos. 4,201,052 and 4,480,527, having a common assignee with the present application, there is disclosed and claimed a hydraulic system for accurately controlling the position and speed of operation of the actuators; which system is simple and easy to make and maintain; and which system is unaffected by change of load pressure of various portions of the system or other actuators served by the same source. In certain high inertial loads such as swing drives on an excavator which utilize rotary actuators, smooth stopping and starting of the load and accurate positioning of the load are very essential.
In U.S. Pat. No. 4,407,122, there is a disclosed a hydraulic control system comprising a hydraulic actuator, a pilot controller and a pump. The actuator includes a movable element and a pair of openings adapted to function alternately as inlets or outlets for moving the element in opposite directions. The pilot controller supplies fluid to the system at pilot pressure and the pump supplies fluid at pump pressure to the actuator. The control system includes a line adapted for connected to each of the openings and a meter-out valve associated with each of the lines for controlling fluid flow from the actuator. The meter-out valves are each selectively pilot operated by pilot pressure from the pilot controller. A meter-in valve means controls fluid flow from the pump to the actuator and is selectively operable by pilot pressure from the pilot controller. In accordance with U.S. Pat. No. 4,407,122, the supply pressure out of the meter-in valve means is sensed and a pressure is applied to the meter-in valve means opposing the pilot pressure which tends to open the meter-in valve means.
In U.S. Pat. Nos. 4,535,809 and 4,964,611, there is disclosed a directional valve of a different construction wherein the directional valve includes a piston that provides a variable orifice and the position of the piston is controlled by a pilot pressure derived from the flow from the main pump which supplies fluid to the directional valve. Such an arrangement can be utilized for metering the flow to an actuator. However, in certain high inertial loads such as swing drives on an excavator, for example, which utilize rotary actuators, smooth starting and stopping of the load and accurate positioning of the load is essential and the system necessitates further damping which is not obtained by the directional valves shown in U.S. Pat. Nos. 4,535,809 and 4,964,611.
Among the objectives of the present invention are to provide a hydraulic control system utilizing valves of the types shown in U.S. Pat. Nos. 4,535,809 and 4,964,611 which provide damping and resultant smooth and accurate positioning of a load; wherein the system requires minimal modification; and which can be achieved at relatively low additional charge.
In accordance with the preferred form of the invention, a hydraulic control system for controlling a hydraulic actuator comprising a pump for supplying fluid to the actuator and a directional valve for controlling the flow of fluid to the actuator. The directional valve includes a poppet piston that together with the housing provides a variable orifice. The position of the poppet piston is controlled by a pilot flow controlled by a pilot valve which may be controlled by a solenoid controller. The flow out of the pilot valve is directed to a position downstream from the directional valve. A pressure compensator is provided between the inlet to the directional valve and maintains a constant pilot flow each pilot valve position regardless of the pressure, differential of the directional valve. The pressure of fluid being applied to the actuator by the directional valve is also sensed and caused to produce a force opposing the opening force applied to the pilot valve to result in a smooth and accurate control of the load.